Downhole detectors of seismic waves are well known in the art. A typical prior art tool includes the following elements in a single housing: sensors, such as geophones, that convert mechanical vibrations into electrical signals; associated electronics; a clamp that fastens the tool to the borehole wall; and a motor that actuates the clamp. These downhole detectors are large with lengths as long as 6 feet and weights as much as 160 pounds. They often have the capacity to clamp in holes with diameters ranging from 5 inches to over a foot.
During acquisition of seismic data, the detector is lowered into a well and clamped at a desired depth. Seismic waves are created by conventional sources and detected by the tool. The tool is then lowered to a new depth, and the process is repeated. In the most common configuration, data can be recorded by only one detector unit at one depth at a time. Recently, new tools have been devised which can record data simultaneously from several detectors locked at different depths as disclosed in European Patent Application 0210925. This prior art tool comprises a seismic detector and a magnetic clamp in an open cradle carrier which can be secured to a cable linking several such devices into an array. The size of the detectors is still a limiting factor on detector spacing, however.
The large size of prior art detectors also limits the frequencies of the seismic signals that can be recorded. Prior art downhole detectors are limited by internal mechanical resonances of the tool and by the force with which the tool is clamped to the borehole wall. Resonances caused by the flexing of a tool body can interfere with the recording of the seismic signals. The larger the tool, the lower the resonant frequencies, and the greater the interference. With a poor clamp, the detector will follow the motion of the borehole wall for low frequencies, but will not couple to the wall at higher frequencies. It is well known that better coupling resulting in detection of higher frequencies is achieved with a greater clamping force-to-weight ratio for the tool. Typical frequency detection limits for prior art geophones are 200 to 300 Hertz.
There are a number of applications, such as that disclosed in U.S. Pat. No. 4,214,226 to Narasimhan et al., which require high-frequency data (1000 Hz) recorded at many different depths in the well. Prior art tools are inadequate to record the higher frequencies for the reasons discussed above. Furthermore, to record this data in a minimal period of time, it is important that data be simultaneously recorded at a number of depths by multiple detectors in the well. There is also an application disclosed in U.S. Ser. No. 430,513 to Krohn which requires multiple detectors spaced at two-foot separations in the well. The prior art tools are too long to be spaced two feet apart. These applications involve operations in uniform wellbores that are often cased, however. Thus, the capacity to record data at a large range of borehole diameters with a single tool configuration is not required.